The accurate occupancy information is crucial to the demand response HVAC control. However, traditional passive infrared (PIR) occupancy sensors can only provide binary results of occupant presence without detecting the number of people in a room. More advanced occupancy detection techniques using CO₂ sensors, cameras, radio-frequency identification (RFID), and computer login events can provide detailed information on occupancy but at higher costs. In this paper, a low-cost bi-directional people counter was developed to determine the occupant number in a single entry room by detecting people entering or leaving the room. The developed people counter is designed to be installed in the doorway with a capability of recording the time, the people moving direction, and the number of people entering or leaving the room. Based on an Arduino microcontroller, the developed people counter consists of two ultrasonic sensors, a real-time clock module, a wireless transceiver, and a SD card module. The direction of people moving was determined by differentiating the triggering sequence of ultrasonic sensors. Laboratory evaluations were conducted to verify the accuracy of the developed people counter by comparing the measured occupancy information with the ground truth recorded manually. The preliminary laboratory results show that the developed people counter can provide satisfactory people counting results with an accuracy of approximately 90%.